Measures for spectral quality in low-voltage X-ray microanalysis.

Characteristic x-ray production with energetic electrons depends strongly on the overvoltage, the ratio of the incident beam energy to the critical excitation energy for the atomic species of interest. Low-voltage x-ray microanalysis (beam energy < or = 5 keV) is especially susceptible to artifacts due to sample charging because the overvoltage is low and even slight charging can strongly affect peak intensities. The Duane-Hunt bremsstrahlung limit is a good diagnostic to detect sample charging. Dynamic charging effects, however, can influence spectra despite an apparently satisfactory Duane-Hunt limit. Dynamic charging effects must be examined by time series experiments, or through use of dynamic energy windows continuously measuring count rates placed across the spectrum. When charging is a problem, conductive surface coatings can eliminate the effects. When pristine surfaces must be examined without coating, the use of a conductive grid can control charging so that useful x-ray spectra can be obtained.